1. Field of the Invention
This invention relates to inkjet printing apparatus and methods for inkjet printing using ink that is curable upon exposure to actinic radiation such as UV radiation. More particularly, the present invention is directed to automated methods and apparatus for controlling the parameters used in inkjet printing.
2. Description of the Related Art
Inkjet printing has increased in popularity in recent years due to its relatively high speed and excellent image resolution. Moreover, inkjet printing apparatus used in conjunction with a computer provides great flexibility in design and layout of the final image. The increased popularity of inkjet printing and the efficiencies in use have made inkjet printing an affordable alternative to previously known methods of printing.
In general, there are three types of inkjet printers in widespread use: the flat bed printer, the roll-to-roll printer and the drum printer. In a typical flat bed printer, the medium or substrate to receive the printed image rests on a horizontally extending flat table or bed. An inkjet print head is mounted on a movable carriage or other type of mechanism that enables the print head to be moved along two mutually perpendicular paths across the bed. The print head is connected to a computer that is programmed to energize certain nozzles of the print head as the print head traverses across the substrate, optionally using inks of different colors. The ink on the substrate is then cured as needed to provide the desired final image.
In roll-to-roll inkjet printers, the substrate to receive the printed image is commonly provided in the form of an elongated web or sheet and advances from a supply roll to a take-up roll. At a location between the supply roll and the take-up roll, a print head is mounted on a carriage that is movable to shift the print head across the substrate in a direction perpendicular to the direction of advancement of the substrate. Known roll-to-roll inkjet printers include vertical printers, wherein the substrate moves in an upward direction past the print head, as well as horizontal printers, wherein the substrate moves in a horizontal direction past the print head.
Drum inkjet printers typically include a cylindrical drum that is mounted for rotational movement about a horizontal axis. The substrate is placed over the periphery of the drum and an inkjet print head is operable to direct drops of ink toward the substrate on the drum. In some instances, the print head is stationary and extends along substantially the entire length of the drum in a horizontal direction. In other instances, the length of the print head is somewhat shorter than the length of the drum and is mounted on a carriage for movement in a horizontal direction across the substrate and parallel to the rotational axis of the drum.
Inks that are commonly used in inkjet printers include water-based inks, solvent-based inks and radiation-curable inks. Water-based inks are used with porous substrates or substrates that have a special receptor coating to absorb the water. In general, water-based inks are not satisfactory when used for printing on non-coated, non-porous films.
Solvent-based inks used in inkjet printers are suitable for printing on non-porous films and overcome the problem noted above relating to water-based inks. Unfortunately, many solvent-based inks contain about 90 percent organic solvents by weight. As solvent-based inks dry, the solvent evaporates and may present an environmental hazard. Although environmental systems may be available for reducing the emission of solvents to the atmosphere, such systems are generally considered expensive, especially for the owner of a small print shop.
Furthermore, inkjet printers using either solvent-based inks or water-based inks must dry relatively large quantities of solvent or water before the process is considered complete and the resulting printed product can be conveniently handled. The step of drying the solvents or water by evaporation is relatively time-consuming and can be a rate limiting step for the entire printing process.
In view of the problems noted above, radiation-curable inks have become widely considered in recent years as the ink of choice for printing on a wide variety of non-coated, non-porous substrates. The use of radiation curing enables the ink to quickly cure (commonly considered as “instant” drying) without the need to drive off large quantities of water or solvent. As a result, radiation curable inks can be used in high speed ink-jet printers that can achieve production speeds of over 1000 ft2/hr (93 m2/hr.) The most common radiation curable inkjet inks are formulated to cure when exposed to actinic radiation, which is radiation having a wavelength in the ultraviolet (“UV”) or visible region of the spectrum.
Inkjet printers that are capable of printing on relatively large substrates are considered expensive. Accordingly, it is desired to use the same printer to impart images to a wide variety of substrates using a wide variety of ink compositions if at all possible. Moreover, it is preferred that each image printed by such printers be of high quality on a consistent basis regardless of the type of substrate and the type of ink used, in view of the time and expense of reprinting the image in instances where the quality of the image is less than desired.
The quality of the image printed by inkjet printers using radiation curable inks is dependent upon the intensity and dosage of the radiation. In general, a lower dosage of radiation provides better adhesion of the ink to subsequent coatings that are applied to the substrate. However, a higher dosage of radiation generally provides an image with better mar and solvent resistance in instances where the ink is not covered by a subsequent coating.
The printer operator often has little assurance that the selected intensity and dosage of radiation will provide the best image quality for any particular combination of ink, substrate and radiation source. Many operators today use a UV meter periodically to check the intensity of emitted radiation. However, such a procedure is cumbersome and time-consuming. Moreover, if the source of radiation is unexpectedly diminished by, for example, a defect or aging of the bulb, the printing process may continue for some time until the operator notices that the quality of the images has been adversely affected.
In view of the foregoing, there is a need in the art for new methods and apparatus of inkjet printing that would consistently enable high quality images to be printed without undue reliance upon the operator's degree of attentiveness. Preferably, such methods and apparatus would be automated and not require a significant amount of operator skill.